German Patent Application No. DE 101 39 142 describes an exhaust-gas treatment system of an internal combustion engine, in which, to reduce the NOx emissions, an SCR (selective catalytic reduction) catalytic converter is used which reduces the nitrogen oxides contained in the exhaust gas to nitrogen using the reagent ammonia. The ammonia is obtained in a hydrolysis catalytic converter, situated upstream from the SCR catalytic converter, from a urea-water solution. The hydrolysis catalytic converter converts the urea contained in the urea-water solution to ammonia and carbon dioxide.
In German Patent Application No. DE 197 39 848, a procedure is described by which the untreated NOx emissions of the internal combustion engine can be at least approximately calculated from known operating parameters of the internal combustion engine. The starting point is a family of characteristics which is defined (spanned) by the load and the speed of the internal combustion engine. In addition, corrections can be provided, for example, as a function of the air ratio lambda.
European Patent Application No. EP 1 024 254 describes an exhaust-gas treatment system of an internal combustion engine, in which an SCR catalytic converter is likewise used for reducing NOx emissions. Ammonia is again provided as a reagent, which is obtained in the exhaust duct from a urea-water solution. The reagent rate is set on the basis of the fuel injection quantity and the speed of the internal combustion engine, as well as on the basis of at least one characteristic of the exhaust gas, e.g. the exhaust-gas temperature.
In European Patent Application No. EP 697 062, a method and a device are described for the controlled introduction of a reagent into an exhaust gas containing nitrogen oxide. An SCR catalytic converter is likewise provided which, as a reagent, needs ammonia that is obtained from a reagent introduced into the exhaust duct upstream of the SCR catalytic converter. At least one operationally-relevant parameter of the exhaust gas, at least one operationally-relevant parameter of a catalytic converter and optionally one operationally-relevant parameter of an internal combustion engine are acquired for determining the untreated NOx emissions of the internal combustion engine. In accordance with the ascertained untreated NOx emissions, an intermediate value is determined for a reagent rate to be stipulated, which is reduced by a reagent rate desorbed by the catalytic converter or is increased by a reagent rate adsorbed by the catalytic converter.
An object of the present invention is to provide a method for operating a catalytic converter used for purifying the exhaust gas of an internal combustion engine and a device for implementing the method which avoid overdosage and underdosage of the reagent.